$ B = \left[\begin{array}{rr}3 & 0 \\ 2 & -2 \\ 1 & 5\end{array}\right]$ $ F = \left[\begin{array}{rr}1 & 3 \\ 5 & 2\end{array}\right]$ What is $ B F$ ?
Explanation: Because $ B$ has dimensions $(3\times2)$ and $ F$ has dimensions $(2\times2)$ , the answer matrix will have dimensions $(3\times2)$ $ B F = \left[\begin{array}{rr}{3} & {0} \\ {2} & {-2} \\ \color{gray}{1} & \color{gray}{5}\end{array}\right] \left[\begin{array}{rr}{1} & \color{#DF0030}{3} \\ {5} & \color{#DF0030}{2}\end{array}\right] = \left[\begin{array}{rr}? & ? \\ ? & ? \\ ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ B$ , with the corresponding elements in column $j$ of the second matrix, $ F$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ B$ with the first element in ${\text{column }1}$ of $ F$ , then multiply the second element in ${\text{row }1}$ of $ B$ with the second element in ${\text{column }1}$ of $ F$ , and so on. Add the products together. $ \left[\begin{array}{rr}{3}\cdot{1}+{0}\cdot{5} & ? \\ ? & ? \\ ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ B$ with the corresponding elements in ${\text{column }1}$ of $ F$ and add the products together. $ \left[\begin{array}{rr}{3}\cdot{1}+{0}\cdot{5} & ? \\ {2}\cdot{1}+{-2}\cdot{5} & ? \\ ? & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ B$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ F$ and add the products together. $ \left[\begin{array}{rr}{3}\cdot{1}+{0}\cdot{5} & {3}\cdot\color{#DF0030}{3}+{0}\cdot\color{#DF0030}{2} \\ {2}\cdot{1}+{-2}\cdot{5} & ? \\ ? & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rr}{3}\cdot{1}+{0}\cdot{5} & {3}\cdot\color{#DF0030}{3}+{0}\cdot\color{#DF0030}{2} \\ {2}\cdot{1}+{-2}\cdot{5} & {2}\cdot\color{#DF0030}{3}+{-2}\cdot\color{#DF0030}{2} \\ \color{gray}{1}\cdot{1}+\color{gray}{5}\cdot{5} & \color{gray}{1}\cdot\color{#DF0030}{3}+\color{gray}{5}\cdot\color{#DF0030}{2}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rr}3 & 9 \\ -8 & 2 \\ 26 & 13\end{array}\right] $